1. Field of the Invention
The present invention relates to a display drive apparatus, a display apparatus and a drive control method thereof, and more particularly to a display drive apparatus which can be applied to a display panel having a plurality of display pixels arranged therein, each display pixel comprising a current controlled type light emitting element which emits a light with a predetermined luminance gradation by supplying a current corresponding to display data thereto, and a display apparatus comprising the display drive apparatus, and a drive control method thereof.
2. Description of the Related Art
There has been conventionally known a light emitting element type display (a display apparatus) comprising a display panel in which display pixels each comprising a current controlled type light emitting element which emits a light with a predetermined luminance gradation in accordance with a current value of a drive current supplied thereto are two dimensionally arranged like an organic electroluminescence element (which will be referred to as an “organic EL element” hereinafter) or a light emitting diode (LED).
In particular, a light emitting element type display adopting an active matrix drive mode has a higher display response speed and no field angle dependence and can realize high luminance/high contrast, high definition of a display image quality, a reduction in power consumption and others as compared with a liquid crystal display apparatus (an LCD) which has greatly spread in recent years. Further, the light emitting element type display comprises light emitting element type display pixels and hence does not require a backlight like a liquid crystal display apparatus. Therefore, the light emitting element type display has very excellent characteristics that a further reduction in thickness and weight is possible, and it has been keenly studied and developed as a next-generation display.
FIG. 18 is a schematic view showing a structural example of a primary part of a light emitting element type display adopting an active matrix drive mode in a prior art.
FIG. 19 is an equivalent circuit diagram showing a structural example of a display pixel applied to the light emitting type display adopting the active matrix drive mode in the prior art.
As shown in FIG. 18, the light emitting element type display adopting the active matrix drive mode in the prior art has a structure comprising: a display panel 110P in which a plurality of display pixels EMP are arranged in a matrix form (n rows×m columns). Each display pixel EMP includes e.g., a later-described pixel drive circuit and a current controlled type light emitting element (e.g., an organic EL element) in the vicinity of each of intersections of a plurality of scanning lines SL and a plurality of data lines DL which are arranged to be substantially orthogonal to each other. A scanning driver 120P is connected to the scanning lines SL of the display panel 110P and sets the display pixels EMP in each row in a selected state by sequentially applying a scanning signal Vsel to each scanning line SL at a predetermined timing. A signal driver 200P is connected with the data lines DL of the display panel 110P, and fetches display data and supplies a gradation current Ipix corresponding to the display data to each data line DL at a predetermined timing.
In such a display, operating states of the scanning driver 120P and the signal driver 200P are controlled by, e.g., a scanning control signal, a data control signal and the like which are generated based on a timing signal supplied from the outside, and a gradation current corresponding to the display data is written in the display pixels in each row set in the selected state by application of the scanning signal. As a result, the respective display pixels emit lights with a predetermined luminance gradation, thereby displaying desired image information.
In such a light emitting element type display adopting the active matrix drive mode, various kinds of drive control mechanisms or control methods which control light emission of the above-described current controlled type light emitting elements have been proposed. For example, there has been known a display comprising a pixel drive circuit which is constituted of a plurality of switching means which control light emission of the light emitting elements as well as the light emitting elements in accordance with each display pixel constituting the display panel.
For example, as shown in FIG. 19, such a pixel drive circuit specifically comprises: a pixel drive circuit DP1; and an organic EL element OEL having an anode terminal connected to a drain terminal of a transistor Tr124 of the pixel drive circuit DP1 and a cathode terminal to which a ground potential is applied. The pixel drive circuit DP 1 includes in the vicinity of each intersection of a pair of scanning lines SL1 and SL2 arranged in parallel with each other and a data line DL: a first transistor Tr121 having a gate terminal connected to the scanning line SL1 and a source terminal and a drain terminal connected to the data line DL and a contact point N121; a second transistor Tr122 having a source terminal and a drain terminal connected to the contact point N121 and a contact point N122; a third transistor Tr123 having a gate terminal connected to the contact point N122, a drain terminal connected to the contact point N121, and a source terminal to which a high power supply voltage Vdd is applied; a fourth transistor Tr124 having a gate terminal connected to the contact point N122 and a source terminal to which the high power supply voltage Vdd is applied.
In this example, in FIG. 19, the first transistor Tr121 comprises an n-channel type field effect thin film transistor, and each of the second to fourth transistors Tr122 to Tr124 comprises a p-channel type field effect thin film transistor. Furthermore, CP1 represents a parasitic capacitance formed between a gate and a source of the third and fourth transistors Tr123 and Tr124.
In the pixel drive circuit DP1 having such a configuration, the organic EL element OEL is subjected to a light emission control as follows by turning on/off the four transistors (switching means) comprising the transistors Tr121 to Tr124 at a predetermined timing.
That is, in the pixel drive circuit DP1, when the display pixel is set in the selected state by respectively applying a high-level scanning signal Vsel1 to the scanning line SL1 and a low-level scanning signal Vsel2 to the scanning line SL2 by the scanning drive 120P, the transistors Tr121, Tr122 and Tr123 are turned on, and the gradation current Ipix corresponding to display data which has been supplied to the data line DL by the signal driver 200P flows through the transistors Tr121 and Tr123. At this moment, since a part between the gate and the drain of the transistor Tr123 is electrically short-circuited by the transistors Tr122 and Tr123 operates in a saturated region. As a result, a current level of the gradation current Ipix is converted into a voltage level by the transistor Tr123, and a predetermined voltage is thereby generated between the gate and the source (a write operation). The transistor Tr124 is turned on in accordance with the voltage generated between the gate and the source of the transistor Tr123, and a predetermined drive current flows to the ground potential from the high power supply voltage Vdd through the transistor Tr124 and the organic EL element OEL, thereby emitting a light from the organic EL element (a light emitting operation).
Subsequently, when, e.g., the high-level scanning signal Vsel2 is applied to the scanning line SL2, the transistor Tr122 is turned off. As a result, the voltage generated between the gate and the source of the transistor Tr123 is held by the parasitic capacitance CP1. Then, when the low-level scanning signal Vsel1 is applied to the scanning line SL1, the transistor Tr121 is turned off. As a result, the data line DL and the pixel drive circuit DP1 are electrically shut off. Consequently, the fourth transistor Tr124 continuously maintains the ON state by a potential difference based on the voltage held in the parasitic capacitance CP1, a predetermined drive current flows to the ground potential from the high power supply voltage Vdd through the transistor Tr124 and the organic EL element OEL, and hence the light emitting operation of the organic EL element OEL continues.
Here, the drive current supplied to the organic EL element OEL through the transistor Tr124 is controlled to have a current value based on a luminance gradation of the display data, and this light emitting operation is controlled to continue for, e.g., one frame period until a gradation current corresponding to the next display data is written in each display pixel.
The drive control method in the pixel drive circuit having such a circuit configuration supplies a gradation current having a specified current value corresponding to the display data to each display pixel (the gate terminal of the third transistor Tr123), and controls the drive current which is passed to the organic EL element based on a voltage held in accordance with the current value, thereby effecting the light emitting operation with a predetermined luminance gradation. Therefore, this method is called a current application mode (or a current specification mode).
Like FIG. 19, there has been also known a drive control method adopting a voltage application mode (or a voltage specification mode) which applies a gradation voltage having a specified voltage value corresponding to display data to each display pixel comprising a pixel drive circuit and an organic EL element, and controls a drive current which is passed to the organic EL element OEL in accordance with the voltage value of the gradation voltage, thereby effecting a light emitting operation with a predetermined luminance gradation. In a pixel drive circuit adopting such a voltage specification mode, irregularities or fluctuations (deterioration) are generated in element characteristics (a channel resistance of a transistor and others) of a switch element which has a selection function or a light emission drive function in dependence on an external environment (an ambient temperature and others), a utilization time and the like, a drive current is affected. Thus, the pixel drive circuit has a problem that desired light emission characteristics (display with a predetermined luminance gradation) cannot be stably realized for a long period in time. Alternatively, when each display pixel is finely formed in order to realize the high definition of the display panel, irregularities in operation characteristics (a current between the source and the drain of the transistor and others) of a switch element constituting the pixel drive circuit become large, and hence an appropriate gradation control cannot be performed. Accordingly, the pixel drive circuit has a drawback that generation of irregularities in light emission characteristics of each display pixel results in deterioration of a display image quality.
On the contrary, the pixel drive circuit adopting the current specification mode comprises a third transistor Tr123 (a current/voltage conversion transistor) which converts a current level of a gradation current corresponding to display data supplied to each display pixel into a voltage level and a fourth transistor Tr124 (a light emission drive transistor) which supplies a drive current having a predetermined current value to the organic EL element OEL. This pixel drive circuit can suppress an influence of irregularities in operation characteristics of the respective transistors Tr123 and Tr124 by setting a current value of a drive current supplied to the organic EL element OEL, and hence has an advantage that the problems of the pixel drive circuit adopting the voltage specification mode can be solved.
However, the pixel drive circuit adopting the current specification mode has the following problems.
In case of writing a gradation current based on display data having a lowest luminance or a relatively low luminance in each display pixel (at the time of low-gradation display), a signal current having a small current value corresponding to a luminance gradation of display data must be supplied to each display pixel.
Here, since an operation of writing a gradation current in each display pixel corresponds to charging a capacitance component which is parasitic on the data line (a retention capacitance constituting a wiring capacitance and a display pixel) to a predetermined voltage, a wiring length of the data line becomes long due to, e.g., an increase in size of the display panel. Moreover, when the number of display pixels connected to this data line is increased, a time required to charges the data line becomes long as a current value of the gradation current becomes small, i.e., as display is effected with a lower gradation, and hence a time required for a write operation with respect to each display pixel is increased. The write operation with respect to each display pixel cannot be completed in a preset write time, and there occurs a so-called insufficient write state in which a stable state (a saturation state) is not reached. As a result, there is a display pixel which cannot emit a light with an appropriate luminance gradation corresponding to display data, resulting in deterioration in a display image quality.
Additionally, when the number of scanning lines arranged on the display panel is increased and a selection period (i.e., a write time) of each scanning line is set short in order to realize the high definition of the display panel, the sufficient write operation with respect to each display pixel cannot be performed as a current value of the gradation current is reduced, and the insufficient write state is generated, which leads to deterioration in a display image quality or a restriction in high definition of the display panel.